1. Field of the Invention
The present invention relates to an apparatus for supplying fuel by means of a fuel supply unit provided with a main housing and accommodated in a fuel tank, which has a fuel pump for an initial fuel flow, a degassing valve and a suction cover arranged at the bottom of the main housing. The fuel pump has an impeller arranged in an impeller housing with a circular passage.
2. Prior Art
An apparatus of this type is known from German Patent Document DE 35 40 260 A1. The known apparatus has a fuel supply unit in a main housing. This sort of fuel supply unit is usually located in a fuel tank, in order to feed fuel from the tank, for example to an internal combustion engine, through an outlet connector of the fuel supply unit by means of a pump intake. Also the pump intake is arranged in a suction cover, which is located in the bottom region of the main housing and the outlet connector is generally arranged beside a degassing valve in the top cover of the main housing. The fuel pump has an electrical drive unit, whose stator and rotor are arranged in the central region of the housing and a pump unit comprising an impeller arranged in an impeller housing is introduced to the lower region of the main housing. The impeller produces suction or low pressure in a circular passage, whereby the fuel is fed.
The degassing valve, which is arranged in an upper region of the main housing, is combined with an overpressure valve in the apparatus for feeding fuel described in German Patent Document DE 35 40 260 A1. The degassing valve thus performs two functions, namely first to remove gas bubbles from the main housing formed by heating of the pump and the impeller and second to permit partial return flow of fuel to the fuel tank when an overpressure exists in the main housing. This type of structure has the disadvantage that gas bubbles are not immediately removed from the region in which they are produced, i.e. in the active region of the fuel pump. These gas bubbles must collect in the upper region of the main housing after the pump runs before they can be released through the degassing valve. Because of that feature the conventional pump has a reduced performance in the starting stage, primarily because the gas collected in the impeller region must first be supplied through the circular passage and then through the entire volume of the main housing to the degassing-overpressure valve before the fuel becomes incompressible during the fuel supply process. That means that the apparatus equipped with a combined degassing-overpressure valve according to German Patent Document DE 35 40 260 A1 has a substantially reduced suction head in its starting stage in comparison to its main operating stage.